In the field of optical communication it is common practice to transmit numerous optical wavelengths separated by small wavelength intervals, often less than 1 nanometer, on a common transmission fiber or free space optical path. Each component wavelength carries a channel of data. At the transmitting and receiving terminal of an optical network, it is necessary to isolate or combine one or a group of these wavelength channels for further processing.
A fixed spacing Fabry Perot interferometer, commonly called an etalon cavity or simply etalon, is capable of mixing or separating closely spaced optical wavelengths by creating optical interference effects as a beam of light experiences multiple reflections between two mirrors etalon cavities have been used in laboratories for spectrographic analysis for over 100 years. A properly designed etalon filter has the very desirable property of very narrow-band transmission, and is well suited to the separation or mixing of closely spaced optical wavelengths that are of interest in optical telecommunications and spectroscopic instruments.
Another characteristic of the etalon is a high sensitivity to temperature changes. The sensitivity is high enough to make the etalon cavity useful for a variety of temperature sensing applications. The temperature sensitivity of the etalon is not desirable for many other applications, including fiber optic telecommunications, where it is desired to maintain constant wavelength transmission characteristics over a wide temperature range. In U.S. Pat. Nos. 5,375,181 and 5,384,877 temperature insensitive etalon filters are described in which thermally reactive support structures change the relative spacing of the 2 reflective elements in an amount sufficient to compensate for thermal effects on the index of refraction of the materials which fill the space of the cavity. These support-actuated designs prove to be excessively long and complex when applied to certain wavelength interleaving applications. Other stabilization techniques include regulating the pressure of a gas between the etalon mirrors, constant temperature ovens, motorized circuits to adjust the mirror spacing, beam angle modifiers, and support materials that apply either radial or axial stress to the etalon cavity. In each case, the thermal compensation modifications add complexity and or size to the device.
It is apparent that etalon devices known in the prior art are not well suited to the conditions of use often found in telecommunications, particularly temperature variations experienced by equipment in the field. This deficiency may lead to diminished utilization of optical communications paths that is far less than the theoretical maximum capacity.
The object of the present invention is to provide an Etalon Filter with a characteristic transmission and reflection property for a single or multiplicity of wavelengths that is stable over a range of temperature changes of the optical element.
It is a further object of the invention to achieve the athermalization of the etalon in a passive manner.
It is yet a further object of the invention to achieve the athermalization of the etalon while having a device size that is substantially the same as an equivalently fabricated non-temperature compensated device.
It is another object of the invention to employ the described etalon to separate a multiplicity of closely spaced optical wavelength channels into 2 or more groups of equally spaced channel subgroups, of which each channel or subgroup of channels have a separate optical path.
It is yet another object of the invention to employ the same described etalon to interleave together a multiplicity of spaced optical wavelengths into a single optical path comprised of more closely spaced optical wavelengths.
The above named objects are achieved by joining two or more different transparent materials to occupy the space between the Fabry Perot mirrors. By selecting the multiple transparent materials according to the teaching outlined below, a non-adjustable etalon can be made with a pre-determined thermal sensitivity of a fixed value that is substantially zero.
It is another object of the invention to provide a single material having a coefficient of thermal expansion and a coefficient of thermal change in refractive index that are functionally opposite. These material properties lead to a beam insertion angle at which the beam refraction is not affected by temperature fluctuations in the material.